Gitstalk is a Web app that allows you to sneak peek into the recent activities of other GitHub users. No Login, No BS. Just enter the username and you are good to fly!
Github User Rank is a simple Chrome Extension that adds rankings straight to GitHub user profiles so you don't have to go anywhere anymore.
Github Profile Visualizer is a tool for visualizing a developer based on their GitHub contributions, field of contributions and a lot more.
OctoSearch is a web app that let's you search the repositories starred by your following on GitHub.